Category: DRM

Yesterday, I told a simplistic story about DeCSS—indeed, the self-same simplistic story about DeCSS that I told my classes this year, and that I suspect a lot of other professors tell their classes—and asked what was wrong with it. The way I put it, if DeCSS really is about preventing only decryption of DVDs, what’s to stop pirates from simply making copies of discs in their encrypted forms? The story simply doesn’t make sense without some additional fact.

Sarah L. (“[T]he CSS disk’s descrambling keys are in sectors that aren’t copied when you make a copy of the disk using a noncompliant player.”) and Bruce Boyden (“[T]he whole scheme depends on licensed drives, which must play by the licensing rules.”) both had important parts of the answer, but what I was looking for is that it is physically impossible to produce CSS-encoded DVDs using home equipment. Sarah’s and Bruce’s points are both true, but even taken together, they wouldn’t explain why DVD Jon or someone else similarly disinclined to care about licensing doesn’t just write a program that writes the descrambling keys to the special sectors. They don’t because they can’t.

To decrypt a CSS-encrypted DVD, you actually need two kinds of keys. One is universal but nominally secret; it’s baked into every DVD player. This is the one that DVD Jon found. The other is different for every disc. But this second key isn’t really secret; it’s written out on the disc, plain as day for anyone to see, in a special “lead-in” sector. Ordinarily, your DVD player reads the public disc key, combines it with its own secret player key, and uses the two together to decrypt the disc contents.

Here’s the twist. There are two ways to make readable DVDs, and they use completely different technology. The large-scale industrial method is to “press” the DVD: that involves encoding the data as a series of tiny three-dimensional bumps on a mold used to stamp a corresponding pattern of pits into metal blanks, which are then encased in a layer of lacquer to make DVDs. This process, as you might imagine, has high fixed costs; the equipment alone will run you upwards of a million dollars. In contrast, the home method is to “burn” the DVD. Here, the blank disc comes from the factory prelacquered and containing an optically sensitive dye on the surface of the metal. Focus the right kind of laser on the dye and its transparency changes. From the perspective of the DVD player that will later read the disc’s patterns of opaque and transparent regions, the results are much the same as if the disc had pits and non-pits. Some areas reflect; others don’t. Ones and zeroes, more or less.

The trick that makes CSS “work” is that you can’t burn lead-in sectors. DVD-Rs (and DVD+Rs) come from the factory with the lead-in sectors zeroed out. Thus, a would-be pirate can easily read an entire encrypted disc, disc key and all, but can only burn back the data portion of the disc, without the disc key. The resulting disc is useless in a standard DVD player; there’s no disc key to be read, which means the player is at a loss in trying to decrypt it. While one could manufacture and distribute home-copied DVDs without having to bust CSS, those DVDs are only going to work on specially-coded software DVD players, not on the mass-produced home players most people have.

That’s why everything does in fact depend on CSS, and why DeCSS really is a big deal. It goes back to the control that the DVD cartel has over their hardware platform, specifically over the manufacturing format of blank media. And that control, in turn, is backed up by patent pools. Yes, you could in theory press (not burn) exact-copies of encrypted discs, or mass-produce your own non-standard blank DVD-Rs with writable lead-in areas, but to do either, you’d need some significant (and hard-to-move) capital, which makes you vulnerable if the cartel comes after you. It’s an ingenious technologico-legal trap.

Tomorrow: Some thoughts on the implications (including responses to comments).